1. Field of the Invention
The present invention relates to an information reproducing apparatus for reproducing information recorded on a magneto-optical information recording medium, and more particularly an information reproducing apparatus utilizing a reproducing method by magnetic wall movement.
2. Related Background Art
In recent years, a high-density magneto-optical information recording medium, effecting information recording and reproduction utilizing a micro light spot, is attracting attention. In the magneto-optical medium, information is recorded in a thin magnetic film in the form of a magnetic domain as being vertical magnetization, utilizing thermal energy provided, for example, by a semiconductor laser, and the recorded information is reproduced by the magneto-optical effect. There is recently an increasing demand for increasing the recording density of such a magneto-optical recording medium. In general, the linear recording density of an optical disk, such as the magneto-optical recording medium, is determined by the wavelength of the laser light of the reproducing optical system and the NA (numerical aperture) of the objective lens. More specifically, as the diameter of the light spot is determined by the wavelength .lambda. of the laser light of the reproducing optical system and the NA of the objective lens, the dimension of the reproducible magnetic domain is approximately limited by .lambda./2NA.
Consequently, for achieving a higher density in the conventional optical disk, it is required to shorten the wavelength of the laser light of the reproducing optical system or to increase the NA of the objective lens. However, there are inevitably limits in such improvements in the wavelength of the laser light and in the NA of the objective lens. For this reason, there have been developed technologies for improving the recording density, by incorporating new technical aspects in the structure of the recording medium or in the reading method. For example, Japanese Patent Laid-open Application No. 3-93058 proposes a technology for reproducing a signal, recorded in a recording holding layer of a multi-layered film containing the recording holding layer and a magnetically coupled reproducing layer, by heating the reproducing layer, after alignment of the direction of magnetization, by laser irradiation and reading the signal recorded in the recording holding layer while it is transferred to the heated area of the reproducing layer. In this method, in comparison with the diameter of the reproducing light spot, the area heated to the signal transfer temperature by such a light spot thereby contributing to the signal detection can be made smaller, so that the intersymbol interference at the signal reproduction is reduced and a magnetic domain of a size smaller than the diffraction limit of light can be reproduced. This method is, however, incapable of providing a sufficient signal-to-noise ratio, because the effective signal detection area becomes smaller than the diameter of the reproducing light spot, whereby the amplitude of the reproduced signal is significantly decreased.
Also, Japanese Patent Laid-open Application No. 6-290496 proposes a reproducing method of the magnetic wall movement type, by irradiating a magneto-optical recording medium, composed of plural laminated magnetic layers, with a light spot thereby transferring the magnetic domain recorded as the vertical magnetization in a recording layer to a reproducing layer, and moving the magnetic wall of the magnetic domain thus transferred to the reproducing layer so as to enlarge the magnetic domain in comparison with that in the recording layer. Such a magnetic wall movement type reproducing method will be explained further in the following. FIGS. 1A to 1D are schematic views showing the magneto-optical recording medium to be employed in such a reproducing method. FIG. 1A is a schematic plan view of the magneto-optical recording medium, and FIG. 1B is a schematic cross-sectional view thereof. In these drawings, there are shown a light spot 60 for information reproduction, and an information track 59 on the magneto-optical recording medium. The recording medium is composed of three magnetic layers, namely first, second and third magnetic layers 62, 63, 64. Arrows in these layers indicate the directions of atomic spins, and magnetic walls 61 are formed at the boundaries of the areas in which the directions of atomic spin are mutually opposite.
FIG. 1C is a chart showing the temperature distribution formed on the magneto-optical recording medium. Such a temperature distribution may be induced on the medium by the light beam itself, irradiated for reproducing the information, or by another heating means which raises the temperature of the medium in front of the spot of the reproducing light beam, so as to form a temperature peak behind such a light spot. It is assumed that the temperature of the medium at a position Xs is at a temperature Ts close to the Curie temperature of the second magnetic layer 63. FIG. 1D shows the distribution of the magnetic wall energy density .sigma.1 of the first magnetic layer 62 corresponding to the temperature distribution shown in FIG. 1C. A slope of the magnetic wall energy density .sigma.1 in the X-direction as shown in FIG. 1D generates a force F1 applied to the magnetic wall in the magnetic layers at the position X, and such a force F1 functions in such a manner as to displace the magnetic walls in a direction toward the lower side of the magnetic wall energy. In the first magnetic layer 62, where magnetic wall coercivity is small and the mobility thereof is high, the magnetic wall in this layer moves easily by such a force F1 if the layer is singly present. However, since the medium temperature is lower than Ts in an area in front of the position Xs (right-hand side in FIG. 1B), the magnetic wall in the first layer 61 becomes fixed at a position corresponding to the position of the magnetic wall in the third magnetic layer 64, by the exchange coupling with the third magnetic layer 64 with a larger magnetic wall coercivity.
If one of the magnetic walls 61 is at the position Xs as shown in FIG. 1B, the exchange coupling between the first magnetic layer 62 and the third magnetic layer 64 is cut off when the medium temperature rises to the temperature Ts close to the Curie temperature of the second magnetic layer 63. As a result, the magnetic wall 61 in the first magnetic layer 62 instantaneously moves, as indicated by an arrow, to an area where the temperature is higher and the magnetic wall energy density is lower. Thus, with the passing of the reproducing light spot 60, the magnetic wall moves as explained above and the atomic spins of the first magnetic layer 62 within the reproducing light spot are all aligned in the same direction. Then, with the movement of the medium, the magnetic wall 61 instantaneously moves and the atomic spins within the light spot are inverted and aligned in the same direction. As a result, the signal reproduced by the light spot always provides a constant amplitude regardless of the magnitude of the magnetic domain recorded in the third magnetic layer 64, whereby there can be avoided the interference of waveforms resulting from the optical limit of diffraction. Consequently, it is rendered possible to reproduce a pit which is smaller than .lambda./2NA representing the resolution limit determined by the wavelength .lambda. of the laser light and the numerical aperture NA of the objective lens, thereby increasing the recording density.
FIG. 2 is a schematic view of an information recording/reproducing apparatus to be employed for reproducing information by magnetic wall movement. In FIG. 2, a semiconductor laser 65 for information recording and reproduction has a wavelength, for example, of 780 nm. A heating semiconductor laser 67 has a wavelength, for example, of 1.3 .mu.m. Both lasers are so positioned that the lights thereof enter the recording medium in a P-polarized state. In general, the laser beam emitted from a semiconductor laser has an oval cross section, and such a laser beam has been shaped as a circular light spot on the recording medium, utilizing, for example, a beam shaping prism and a substantially circular aperture. The laser beams emitted from the semiconductor lasers 65, 67 are shaped into beams of substantially circular cross sections by unrepresented beam shaping means, and are, respectively, converted into parallel light beams by collimating lenses 66, 68.
There are also provided a dichroic mirror 69, which is so designed as to transmit 100% of the light of 680 nm and to reflect 100% of the light of 1.3 .mu.m, and a polarizing beam splitter 70, which is so designed as to transmit 70 to 80% of the P-polarized light and to reflect almost 100% of the S-polarized light, which is perpendicular to the P-polarized light. The parallel light beams obtained by conversion by the collimating lenses 66, 68 enter an objective lens 71 through the dichroic mirror 69 and the polarizing beam splitter 70 in such a manner that the light beam of 780 nm occupies a larger area in the aperture of the objective lens 71 while the light beam of 1.3 .mu.m occupies a smaller area in the aperture. Consequently, even with the same objective lens 71, the numerical aperture of the lens acts weaker on the light beam of 1.3 .mu.m, whereby the light spot of 1.3 .mu.m becomes larger than that of 780 nm on the recording medium 73. The light beams reflected from the recording medium are again formed by the objective lens 71 into parallel light beams, which are reflected by the polarizing beam splitter 70 as light beams 72. The light beams 72 enter an unrepresented optical system, then separate by wavelength, and regenerate a servo error signal and a reproduced information signal.
In the following, there will be explained, with reference to FIGS. 3A and 3B, the relationship between the recording-reproducing light spot and the heating light spot on the recording medium in the apparatus shown in FIG. 2. In FIG. 3A, there are shown a recording-reproducing light spot 74 of a wavelength of 780 nm, a heating light spot 75 of a wavelength of 1.3 .mu.m, a magnetic wall 76 of a magnetic domain recorded on a land 77, a groove 78, and an area 79 in which the temperature is raised by the heating light spot 75. A sloped temperature distribution as shown in FIG. 3B can be formed on the moving recording medium, by coupling the recording-reproducing light spot 74 and the heating light spot 75 on the land 77 between the grooves 78, and the formation of such a sloped temperature distribution provides the ability to execute the reproduction by magnetic wall movement as explained in relation to FIGS. 1A to 1D.
On the other hand, a method of utilizing an oval-shaped light spot for reproducing the information is disclosed, for example, in Japanese Patent Laid-open Application No. 8-180492. In this method, there is formed an oval-shaped light spot on the optical disk, without the optical system for converting the condensed light spot into a circular form, in such a manner that the direction of a longer axis coincides with the direction of the track. Consequently, there can be achieved effects of dispensing with the optical system for converting the light spot into a circular form, thereby enabling inexpensive manufacture of the optical head, and reducing the size of the light spot in the direction perpendicular to the track, thereby suppressing the influence of crosstalk noise and disk noise resulting from the shape of the groove.
Also, Japanese Patent Laid-open Application No. 5-94624 discloses a method of information reproduction by forming ultra-resolution light spots with an optical filter, irradiating plural marks with the oval-shaped main light spot and detecting the interference pattern from such plural marks.
In the above-mentioned conventional information reproducing methods by magnetic wall movement, the shape of the recording-reproducing light spot is not optimized, and there are margins for improvement in the signal-to-noise ratio and for suppression of the leakage of the information signal from the neighboring tracks.
Also, in the conventional method of the above-mentioned Japanese Patent Laid-open Application No. 8-180492 utilizing the oval-shaped light spot for reproducing the information, since reproduction of information by magnetic wall movement is not utilized, the resolving power in the direction of a track is deteriorated if the longer axis of the oval-shaped light spot becomes larger, whereby the signal-to-noise (C/N) ratio becomes lower. For this reason, the ratio of the longer axis/shorter axis of the oval-shaped light spot is optimally within a range of 1.1 to 1.3. Also, it is described that the size of the light spot can be made smaller in the direction perpendicular to the track, but, as long as the numerical aperture of the objective lens remains the same as in the prior art, the size of the light spot remains unchanged in the direction perpendicular to the track, but becomes smaller only in the direction along the track. Consequently, an appreciable decrease of the crosstalk cannot be expected for the smallest mark (on the order of 1/3 to 1/2 of the light spot) employed in the conventional recording-reproducing method.
Furthermore, the above-mentioned Japanese Patent Laid-open Application No. 5-94624, utilizing the ultra-resolution light spot prepared with the optical filter, does not describe the reproduction of information by magnetic wall movement nor the crosstalk.